The present state-of-the-art satellite paging systems comprise a fixed position antenna mounted on a satellite in synchronous orbit. Typically in such systems, the antenna is fixed so that the spot or the "footprint" of the signal beam covers the portion of the globe which is "visible" to the satellite; that is, approximately one-third of the earth's surface.
The ground level strength of the signal transmitted by the antenna in these prior art systems is fairly uniform across the covered area; however, the signal strength is very weak. This creates a problem, even for the highest powered synchronous satellites, since the ground level signal is too weak to allow for in-building paging coverage. For example, for a synchronous satellite with a radiated power of 400 watts, a state-of-the-art selective call paging receiver using the POCSAG paging protocol at a bit rate of 512 bits per second has been observed to have only 2 dB of signalling margin. That is, the average signal strength on the ground is only 2 dB above the minimum level required to activate a paging device. Thus, considering that most buildings attenuate RF paging signals by 20 dB, these systems could not be used to signal paging users inside buildings.
One solution to the problems with the current system would be to reduce the bandwidth of the paging receiver by lowering the data rate to 60 baud or less. However, this makes the paging system extremely expensive and impractical in view of the small number of users that can be serviced by the paging system. That is, the low bit rate could only support a small volume of paging traffic.
The co-pending U.S. patent application mentioned previously provides a significant advancement over the current art in that it describes a satellite paging system that uses an antenna with a smaller "footprint" or beam-width to achieve significantly higher ground level signal strengths, and the antenna beam is steered to cover different geographical areas in a repetitive pattern to achieve wide area coverage. The combination of the resultant higher ground level signal strength, the scanning motion of the beam over the surface of the earth, and a pager battery saver technique that turns the paging receivers ON and OFF in synchronism with the scanning movement of the beam, allows a signalling data rate of either 512 baud or 1200 baud to be used and offers the first practical approach for implementing a very wide area or global satellite based paging system.
However, even the improved system described in the copending application has several limitations. First of all, even the 512 baud or 1200 baud signalling rate it can achieve is not high enough to support the very large number of users that can be expected to eventually want global paging service. Secondly, the system can only handle regional population differences by varying the length of time the scanning beam is positioned to cover a given region. Finally, the system has no way of taking advantage of the fact that most of the population of the developed countries resides in densely populated cities.
Thus, it is an object of the present invention to provide an improved satellite signalling system with greater message throughput.
It is another object of the invention to provide a satellite signalling system that can accommodate regional differences in population density.
It is yet another object of the invention to provide a satellite signalling system that can selectively support a higher data signalling rate.
It is still another object of the invention to provide a satellite signalling system that can support a sufficient number of users as to render the system economically viable.